Giant strain-sensitivity of acoustic energy dissipation in solids containing dry and saturated cracks with wavy interfaces

TL;DR

This paper investigates how wavy crack interfaces in solids lead to extremely high strain sensitivity of acoustic energy dissipation, explaining observed tidal effects on seismo-acoustic signals without requiring unrealistically thin cracks.

Contribution

It introduces a revised dissipation mechanism considering wavy asperities of crack surfaces, significantly enhancing strain sensitivity predictions in heterogeneous solids.

Findings

Wavy crack asperities drastically alter relaxation frequencies.

Mechanism explains high strain sensitivity of seismo-acoustic loss.

Accounts for tidal modulation of seismic signals.

Abstract

Mechanisms of acoustic energy dissipation in heterogeneous solids attract much attention in view of their importance for material characterization, nondestructive testing, and geophysics. Due to the progress in measurement techniques in recent years it has been revealed that rocks can demonstrate extremely high strain sensitivity of seismo-acoustic loss. In particular, it has been found that strains of order $10^{-8}$ produced by lunar and solar tides are capable to cause variations in the seismoacoustic decrement on the order of several percents. Some laboratory data (although obtained for higher frequencies) also indicate the presence of very high dissipative nonlinearity. Conventionally discussed dissipation mechanisms (thermoelastic loss in dry solids, Biot and squirt-type loss in fluid-saturated ones) do not suffice to interpret such data. Here, the dissipation at individual cracks is revised taking into account the influence of wavy asperities of their surfaces quite typical of real cracks, which can drastically change the values of the relaxation frequencies and can result in giant strain sensitivity of the dissipation without the necessity to assume the presence of unrealistically thin (and, therefore, unrealistically soft) cracks. In particular, these mechanisms suggest interpretation for observations of pronounced amplitude modulation of seismo-acoustic waves by tidal strains.